For producing multicoat systems having a primer comprising an electrodeposition coating material with a coat situated above it, the process of what is known as wet-on-wet application of electrodeposition coating material and at least one further coat is known, for example, from the patent applications EP 0 817 684 A1, EP 0 639 660 A1, EP 0 595 186 A1, EP 0 646 420 A1 or DE 41 26 476 A1. The coating materials which are applied wet-on-wet may be in liquid (aqueous, conventional or powder slurry) or powder form. The coating materials may be pigmented and unpigmented and may be used to produce surfacers or functional coats (pigmented) or clearcoats (unpigmented), but especially to produce surfacers.
During the implementation of the wet-on-wet processes, the applied electrodeposition coating film is generally predried prior to the application of the next coating material. This is generally done under conditions in which water and solvents are largely evaporated from the electrodeposition coating film. This procedure is environmentally and economically advantageous and, moreover, generally produces better-quality coatings.
Nevertheless, it is possible again and again to observe problems with the surface appearance (i.e., appearance of the overall system including clearcoat). These problems are manifested, for example, in the values of a longwave/shortwave wavescan (light reflection) which gives a value for the amount of scattered light. The flow of the coated material, as well, in many cases fails to meet requirements.
Attempts have been made to solve these problems, in a very wide variety of ways.
For example, in the process according to the German Patent Application DE 41 26 476 A1, the use of electrodeposition coating materials is restricted to those which on curing have a baking loss of less than 10%. However, this imposes severe restrictions on the user in the selection of suitable electrodeposition coating materials.
The process according to the European Patent Application EP 0 646 420 A1 uses electrodeposition coating materials and powder coating materials whose baking temperatures are harmonized with one another. Thus, the interval of the minimum baking temperature of the second coat (powder coat) should lie above the interval of the first coat (electrodeposition coat), or the intervals should overlap such that the lower limit of the interval of the minimum baking temperature of the second coat lies above the lower limit of the interval of the electrodeposition coat. In other words, the electrodeposition coating material has a baking temperature which is lower than the baking temperature of the powder coating material. Despite this adaptation of the baking temperatures, problems of appearance and of flow continue to occur. Moreover, extensive flaking may occur on stone impact.
Accordingly, the attempts to solve the problems stated have essentially concentrated on selecting only electrodeposition coating materials having a low volume shrinkage or on adapting to one another the baking temperatures of the electrodeposition coating film and the second coating film.